In electrophotography an image comprising an electrostatic field pattern, usually of non-uniform strength (also referred to as an electrostatic latent image), is formed on an insulative surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the insulative surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise radiation-induced dissipation of the initially uniform electrostatic field is brought about by the creation of electron/hole pairs, which are generated by a material (often referred to as a charge-generation or photoconductive material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initial uniform electrostatic field and the types of materials included in the electrophotographic element, part of the charge that has been generated, i.e., either the holes or the electrons, migrate toward the charged insulative surface of the element in the exposed areas and thereby cause the imagewise dissipation of the initial field. What remains is a non-uniform field constituting the electrostatic latent image.
Such elements contain material which facilitates the migration of generated charge toward the oppositely charged surface in imagewise exposed areas in order to cause imagewise field dissipation. Such material is often referred to as a charge-transport material.
One type of well-known charge-transport material comprises a triarylamine. The term, "triarylamine," as used herein is intended to mean any chemical compound containing at least one nitrogen atom that is bonded by at least three single bonds directly to aromatic rings or ring systems. The aromatic rings or ring systems can be unsubstituted or can be further bonded to any number and any types of substituents. Such triarylamines are well known in the art of electrophotography to be very capable of accepting and transporting charges generated by a charge-generation material.
Among the various known types of electrophotographic elements are those generally referred to as multiactive elements (also sometimes called multilayer or multi-active-layer elements). Multiactive elements are so named, because they contain at least two active layers, at least one of which is capable of generating charge in response to exposure to actinic radiation and is referred to as a charge-generation layer (hereinafter referred to as a CGL), and at least one of which is capable of accepting and transporting charges generated by the charge-generation layer and is referred to as a charge-transport layer (hereinafter referred to as a CTL). Such elements typically comprise at least an electrically conductive layer, a CGL, and a CTL. Either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CGL or CTL. Of course, the CGL comprises at least a charge-generation material (a photoconductor); the CTL comprises at least a charge-transport material; and either or both layers may additionally comprise a film-forming polymeric binder.
Among the known multiactive electrophotographic elements, are those which are particularly designed to be reusable and to be sensitive to imagewise exposing radiation falling within the visible and/or infrared regions of the electromagnetic spectrum. Reusable elements are those that can be practically utilized througha plurality (preferably a large number) of cycles of uniform charging, imagewise exposing, development and/or transfer of electrostatic latent image or toner image, and erasure of remaining charge, without unacceptable changes in their performance. Visible and/or infrared radiation-sensitive elements are those that contain a charge-generation material which generates charge in response to exposure to visible and/or infrared radiation. Manu such elements are well known in the art.
For example, some reusable multiactive electrophotographic elements which are designed to be sensitive to visible radiation are described in U.S. Pats. Nos. 4,578,334 and 4,719,163, and some reusable multiactive electrophotographic elements which are designed to be sensitive to infrared radiation are described in U.S. Pats. Nos. 4,666,802 and 4,701,396.
Many known reusable multiactive electrophotographic elements sensitive to visible or infrared radiation also employ triarylamine charge-transport materials in their CTL. In those elements the triarylamine is dispersed or dissolved in a film-forming polymeric binder that forms the CTL. Such elements are described, for example, in the four U.S. patents noted above. Those patents teach many polymers as having utility as film-forming binders for CTL's. Among the many polymers so described, are polycarbonates, such as poly[2,2-bis(4-hydroxyphenyl)propane carbonate] (commonly referred to as bisphenol A polycarbonate), and polyesters. Elements containing such components fairly adequately perform their intended functions, and, in the case of the elements described in the four U.S. patents noted above, have some very important advantages over other known elements. However, the present inventors have recognized some significant drawbacks associated with such elements.
For example, if the CTL comprises a triarylamine in a bisphenol A polycarbonate film, a significant problem may arise. The problem can occur when the CTL has been adventitiously exposed to ultraviolet radiation (i.e., radiation of a wavelength less than about 400 nanometers, which, for example, forms a significant portion of the radiation emitted by typical fluorescent room lighting). This can occur, for example, when the electrophotographic element is incorporated in a copier apparatus and is exposed to typical room illumination during maintenance or repair of the copier's internal components. The problem, which we will refer to as a UV-fogging problem, is manifested as a buildup of residual potential within the electrophotographic element over time as the element is exercised through its normal cycles of electrophotographic operation after having been adventitiously exposed to ultraviolet radiation.
For example, in normal cycles of operation such an element might be initially uniformly charged to a potential of about -500 volts, and it might be intended that the element should then discharge, in areas of maximum exposure to normal imagewise actinic visible or infrared exposing radiation, to a potential of about -100 volts, in order to form the intended latent electrostatic image. However, if the electrophotographic element has been adventitiously exposed to ultraviolet radiation, there will be a buildup of residual potential that will not be erased by normal methods of erasing residual charge during normal electrophotographic operation. For example, after about 500 cycles of operation, the unerasable residual potential may be as much as -200 to -300 volts, and the element will no longer be capable of being discharged to the desired -100 volts. This results in false images being formed in areas of maximum imagewise exposure that should correspond to highlights, i.e., areas of no image density in the original image being copied. In effect, the element has become no longer reusable, after only 500 cycles of operation.
While the mechanism of the UV-fogging problem is not presently understood, the present inventors theorize that the problem may be caused by a chemical change in the triarylamine charge-transport material, induced by absorption of ultraviolet radiation. This is evidenced by an observed color change in the CTL after exposure to ultraviolet radiation. It would be desirable to be able to avoid or minimize this UV-fogging problem.
On the other hand, the present inventors have recognized that, if the electrophotographic element comprises a CTL, wherein the triarylamine is contained in a binder film of a polyester, the UV-fogging problem does not arise. The present inventors theorize that this may be because the polyester absorbs more ultraviolet radiation than does a biphenol A polycarbonate, and thus prevents some of the ultraviolet radiation from being absorbed by the triarylamine in significant enough amounts to cause the chemical change that leads to the UV-fogging problem, and/or the polyester or some complex of the polyester with the triarylamine may otherwise quench or prevent the UV-induced chemical change from occurring.
Unfortunately, such elements having a polyester as their CTL binder exhibit another drawback recognized by the present inventors; namely, they have significantly lower sensitivity to actinic visible or infrared radiation (sometimes referred to as lower speed) than do elements that utilize bisphenol A polycarbonate as their CTL binder. For example, in some cases the exposure to actinic radiation necessary for discharging the initial uniform electrostatic field from -500 to -100 volts (sometimes referred to as the 100-volt speed), is about 75 percent more when a polyester is the CTL binder, compared to when bisphenol A polycarbonate is the CTL binder. This is a very significant difference in terms of high speed copiers; i.e., the copier using polycarbonate as the CTL binder can make more than 5 exposures in the same time it takes the copier with the polyester CTL binder to make 3 exposures. It would, of course, be desirable to retain this speed advantage of the polycarbonate.
It thus becomes evident that there is a need for a reusable visible and/or infrared-sensitive electrophotographic element that avoids or minimizes the UV-fogging problem of elements utilizing a polycarbonate CTL binder, while at the same time avoiding or minimizing the speed loss inherent in elements utilizing polyester CTL binder. The present invention meets this need.